The present invention relates to a golf club shaft.
A conventional golf shaft is designed so that its torsional rigidity or bending rigidity is distributed to gradually increase as a position goes from a shaft front end portion toward a shaft grip portion, and takes the maximum in a shaft end portion on the grip side (a rear end portion of a grip portion). For example, Japanese Patent Unexamined Publication No. Hei-5-337223 discloses a golf club shaft in which the ratio (Ta:Tb) of torsional rigidity (Ta) in the rear end portion of the grip portion to torsional rigidity (Tb) in the shaft front end portion is defined so as to fall in a range of from 1:1 to 4:1.
In the above-mentioned golf club shaft, however, a question is simply put merely on the torsional rigidity (Ta) in the rear end portion of the grip portion and the torsional rigidity (Tb) in the shaft front end portion, while the total torsional rigidity of the shaft is not taken into consideration. It is therefore impossible to satisfy user's various requests in points such as handling performance, handling stability, directional stability, soft hitting sense, etc.
The characteristic required also for a golf club shaft is that the golf club shaft is light in weight so as to swing out easily, while the flying distance can be increased. To satisfy this request, it has been advanced to study a golf club shaft using fiber-reinforced prepreg. Particularly, in order to extend the flying distance, it has been studied to make the flexibility larger on the head side of the shaft than on the grip side.
A golf club shaft which can extend the flying distance is disclosed in Japanese Patent Post-Examination No. Sho-60-40309. In this golf club shaft, a small-diameter portion is provided in a head-side half of the whole length of the shaft so that a kick point which determines the behavior of the shaft appears in this small-diameter portion. Thus, the flexibility can be increased, and the flying distance can be extended.
However, since the small-diameter portion is provided in the head-side half of the whole length of the shaft in the golf club shaft, most part of the shaft is thick. Accordingly, there is a disadvantage that it is difficult to swing out sharply. In addition, only portion near the head side from the small-diameter portion is bent easily in the shaft, so that there is another disadvantage that it is easily broken at the small-diameter portion.